In the prior art, ceramics containing lead titanate or lead zirconate titanate (PZT) as a main component are known as piezoelectric materials. Piezoelectric ceramic compositions further containing a second component, an optional third component and various additives are known to exhibit improved piezoelectric and electric properties.
Piezoelectric compositions having sufficient piezo-electric properties for use in ultrasonic vibrators, filters, and piezoelectric transformers are obtained by adding manganese oxide and cobalt oxide to lead zinc-niobate titanate zirconate as described in JP-B 18400/1979. Similar compositions wherein part of lead is replaced by strontium, barium or the like are disclosed in JP-A 154682/1987. These compositions are now used in a variety of applications.
While piezoelectric transformers have long been studied, they now draw more attention because of their advantages of compactness (especially thickness reduction), weight reduction, high efficiency, and low noise as compared with electromagnetic transformers.
In general, piezoelectric transformers are in the form of a rectangular piezoelectric ceramic plate structured such that one half is polarized in a thickness direction to constitute a primary or input section and the other half is longitudinally polarized to constitute a secondary or output section. When an ac electric field of one or one half wavelength resonance is applied across the piezoelectric transformer of this structure, the electrical energy is converted into vibration energy in the low impedance primary section. This vibration energy is conveyed to the high impedance secondary section where it is converted into electrical energy to develop a high voltage.
The piezoelectric ceramics for use in piezoelectric transformers are required to have a high electromechanical coupling coefficient (k.sub.31) and a large mechanical quality factor (Qm). If they are small, the loss becomes greater and the conversion efficiency becomes lower. More particularly, the heat generated upon high-power driving increases, which invites a further reduced conversion efficiency and a reduced voltage step-up ratio. In the piezoelectric ceramics, defects such as voids are closely correlated to mechanical strength. If piezoelectric ceramics having a number of defects such as voids are used in piezoelectric transformers and other devices that are driven at a large amplitude, not only the ceramics are broken by impact, but breakage also occurs at the point where stresses concentrate, known as the nodal point, when the input voltage is increased to excite vibration at a large amplitude. Therefore, the piezoelectric ceramics for use in piezoelectric transformers are required to have a high mechanical strength. Also, the piezoelectric ceramics for use in ultrasonic motors are required to have a high electromechanical coupling coefficient (k.sub.31) and a high mechanical quality factor (Qm) as well as a high mechanical strength.
Among the piezoelectric ceramics for piezoelectric transformers, for example, JP-B 18400/1979 discloses a composition comprising a lead zinc-niobate titanate zirconate composition represented by Pb (Zn.sub.1/3 Nb.sub.2/3).sub.x Ti.sub.y Zr.sub.z O.sub.3 wherein x+y+z=1, 0.01.ltoreq.x.ltoreq.0.5, 0&lt;y.ltoreq.0.75, and 0&lt;z .ltoreq.0.75, as a main component and further containing 0.01 to 5% by weight of manganese oxide (MnO.sub.2) and 0.1 to 5% by weight of cobalt oxide (CoO). JP-A 112542/1994 discloses a lead zirconate titanate ceramic composition comprising PbO, ZrO.sub.2 and TiO.sub.2 and similar ceramic compositions wherein zirconium and titanium are replaced in part by antimony and niobium.
However, in the piezoelectric ceramic of the composition described in the above-referred JP-B 18400/1979, it is impossible to increase both the electromechanical coupling coefficient (k.sub.31) and mechanical quality factor (Qm) and hence, to provide a fully high conversion efficiency. No improvement in mechanical strength is expected.
Intending to improve the mechanical strength of piezo-electric ceramics, the above-referred JP-A 112542/1994 proposes that the raw material powder is finely divided to a smaller particle size, specifically a specific surface area of at least 10 m.sup.2 /g and calcined at a temperature below 650.degree. C. so that the resulting element may have a grain size of up to 1 .mu.m. However, the finely divided raw material powder is difficult to handle and the addition of a comminuting step increases the cost of manufacture. The piezoelectric ceramic of the composition described in JP-B 18400/1979 is low in both electromechanical coupling coefficient (k.sub.31) and mechanical quality factor (Qm) and hence, conversion efficiency.
It is known to increase the mechanical strength of piezoelectric ceramics by utilizing a hot press. The use of a hot press prolongs the manufacturing process as compared with conventional firing and requires an expensive equipment, leading to an increased cost of manufacture.